The invention relates to a pressure or differential pressure sensor 1 as well as a method for producing such a sensor.
Measuring pressure or differential pressure in fluids forms one of the most important tasks in the observation of technical installations. Nowadays instead of manometers, sensors are increasingly applied which, depending on the pressure or differential pressure, produce or modify an electrical signal. Such sensors are not only preferable with respect to the constructional size and measurement accuracy, but also particularly allow a simple integration in electronic control, observation and likewise.
Sensors according to the preamble type are for example known from U.S. Pat. No. 4,686,764 and U.S. Pat. No. 4,732,042. The membranes of these known sensors, which are impingable on one side by changing pressure, or, in the case of application as a differential pressure sensor, on both sides, are comprised of silicon. Measuring elements in the form of resistances are deposited on these silicon membranes, these resistances able to be contacted and switched as measuring bridges in the usual manner by way of strip conductors likewise deposited on the membrane on the edges thereof. By way of detuning the measuring bridge, the deflection of the membrane and thus the prevailing pressure or the difference in prevailing pressures may be determined. Such switching arrangements have been known for some time (strain gauges).
Since the change in resistance of the measuring resistances deposited on the membrane is also dependent on temperature, as a rule, directly next to the deflection region of the membrane, a compensation resistance, via which a temperature compensation of the measurement is effected, is deposited on said membrane.
Although the membrane composed of silicon with the measuring element deposited thereon allows a measurement with comparatively high accuracy within a small space, the membrane and the electronics located thereon are sensitive, so that they require protection, via which direct contact of the fluid with the membrane is prevented. According to the previously mentioned US patents, such a protection is formed by a layer of gel deposited on the membrane, whereby the gel layer is arranged in a constant volume space and covered by a rubber membrane. The fluid whose pressure is to be measured thus contacts this rubber membrane, the forces directed thereon are transfered through the rubber membrane to the gel and then to the silicone membrane. It is also known to dispense with the rubber membrane and to provide the silicon membrane with a protective gel and to arrange it in a housing which is only connected to the fluid, whose pressure is to be measured, in a pressure conducting manner by way of a comparatively small opening.
The disadvantage of these arrangements is on the one hand the high constructional effort in protecting the membrane and on the other hand the deterioration of the measuring characteristics due to the protection. The thicker and tougher this protective layer actually is, the more inaccurate becomes the measurement. On the other hand the protective layer must ensure a reliable and lasting protection of the silicon membrane, since direct contact with the fluid as a rule leads to permanent damage of the membrane and thus leads to the destruction of the sensor. Furthermore with the known designs, the contacting of the measuring elements located on the membrane is complicated. As a rule it is effected via individual bare wires which are connected via bonds to a strip conductor on the membrane and a strip conductor on a circuit board which is arranged at a slight distance to the membrane and comprises further electronic components where appropriate. The wires must be led out of the pressure space into the other part of the sensor in a pressure tight manner.